This invention relates to a method of coating involving applying to a substrate a pigmented basecoating composition containing a thermoplastic, non-crosslinked, film-forming polymer to form a basecoat and coating the basecoat with one or more applications of a transparent, crosslinking, topcoating composition containing a crosslinkable, film-forming material and a crosslinking agent for the crosslinkable, film-forming material to form a transparent topcoat (a so-called "color plus clear" type method of coating).
A number of known "color plus clear" methods of coating for providing automotive quality finishes, particularly in automotive refinishing applications, utilize two-package compositions based on hydroxyl-functional components and curing (crosslinking) agents containing isocyanate groups. However, the use of isocyanate-functional materials often requires that precautions be taken with respect to the handling and use of the isocyanates based on toxicity considerations. Such precautions can be relatively burdensome particularly when the coating compositions are utilized in environments not involving controlled factory conditions as exist, for example, in plants producing new automotive vehicles. For example, the application of automotive refinishing compositions tends to be done in refinishing shops under conditions which are not nearly as well controlled as those existing in automotive plants which manufacture original equipment. Accordingly, there is a need for high quality coating methods which are not based on the utilization of isocyanate curing agents in at least one, and preferably in both, of the pigmented basecoating and transparent topcoating compositions.
Irrespective of toxicity considerations with respect to the use of isocyanate crosslinking agents, in general there are problems associated with the use of topcoats based on crosslinking materials over basecoats based on non-crosslinked, thermoplastic film-forming polymers (for example, acrylic lacquer basecoats) in "color plus clear" methods of coating as utilized, for example, in automobile refinishing applications. One problem involves lack of repairability of the resulting composite coating. If, for example, a hardened composite film, resulting from a "color plus clear" application method during original equipment manufacture, contains imperfections, and thus needs to be sanded and repaired, it is critical that the composite film be readily susceptible to being repaired. Likewise, when the protective coating, for example on an automobile, becomes damaged during use of the article, it is important that the coating be readily susceptible to repair. The usual manifestation of a repairability problem involves lifting, wrinkling, etc. of the film in the area of the repair where the new coating is applied over the old one, such as in the "feather edge" area of repair where the new coating overlaps the old coating.
This "repairability" problem does not tend to occur when the composite film consists of a lacquer type topcoat over a lacquer type basecoat, but rather when the composite film is made up of a crosslinked topcoat over a non-crosslinked (e.g., lacquer type) basecoat. The present invention is directed, in part, to providing a "color plus clear" method of coating employing a non-crosslinked, thermoplastic film-forming polymer in the basecoating composition and a crosslinking, film-forming material in the topcoating composition which results in a hardened composite film which has excellent repairability characteristics. Other objects of the invention will become apparent to the reader infra.